1. Field of the Invention
The present invention relates to a driving apparatus which uses a member for generating an extension/contraction displacement as a driving source. The present invention relates more particularly to a driving apparatus which uses an electromechanical transducer such as a piezoelectric element, and to a driving apparatus for use in accurately driving probes of, for example, an X-Y driving table, an image pickup lens of a camera, and a scan type tunneling electron microscope.
2. Description of the Related Art
Conventionally, a variety of types of driving apparatuses which use piezoelectric elements have been proposed.
FIG. 1 is an exploded perspective view of a prior art linear movement type driving apparatus, while FIG. 2 is an overall perspective view of the driving apparatus shown in FIG. 1. A driving apparatus 10a is provided with a fixed member 24, a piezoelectric element 22, a driving shaft 26 slidably supported in its axial direction by the fixed member 24 and a driving unit 28 to be connected to a stage which moves with a driven member (not shown) of, for example, a component placed on it. The piezoelectric element 22 is a sort of electromechanical transducer, the length of which is varied by voltage application. The piezoelectric element 22 has one end surface 22a affixed to the fixed member 24 in the direction in which it extends or contracts and has the other end surface 22b affixed to one axial end surface 26a of the driving shaft 26 in the direction in which it extends or contracts. The driving unit 28 is provided with a leaf spring 28a, and the driving unit 28 is frictionally coupled with the driving shaft 26 by the urging force of this leaf spring 28a. In this driving apparatus 10a, the driving unit 28 is moved in a specified direction along the driving shaft 26 when the driving shaft 26 is vibrated in a reciprocating manner in the axial direction by applying to the piezoelectric element 22 a periodic pulse voltage having a sawtooth waveform, for example.
The driving shaft 26 of the driving apparatus 10a is required to have a high modulus of longitudinal elasticity and a high surface hardness on the frictional surface in the extension/contraction direction of the electromechanical transducer, or the piezoelectric element 22. The driving shaft 26 is also required to have a light weight and a smaller specific gravity for sufficient displacement of the piezoelectric element 22. In general, as for the characteristics of the driving frictional member, a modulus of longitudinal elasticity of not smaller than 8000 kg/mm.sup.2 and a surface hardness of not smaller than 300 Hv are required under the practical driving conditions. Conventionally, in order to obtain the characteristics as described above, a carbon fiber composite resin in which carbon fibers are aligned in the extension/contraction direction of the piezoelectric element 22 has been used as the driving frictional member.
However, it is required to optimize the shape of the driving frictional member according to the type of the driving apparatus and the object to be driven. In the case of the carbon fiber composite resin in which carbon fibers are aligned in the extension/contraction direction of the piezoelectric element 22, when its cross-section shape is significantly changed in the middle as in, for example, a driving frictional member 26' shown in FIG. 3, it is difficult to obtain the required characteristic since the carbon fibers are cut or changed in density at the portion in which the cross-section shape is changed or other problems may also occur. That is, the driving frictional member using the carbon fiber composite resin has reduced freedom of shape.
Furthermore, in the driving apparatus 10a, the driving characteristics change depending on the mass of the object to be driven and the frictional force generated on the frictional surface. Therefore, the optimum frictional force has conventionally been obtained by repeating actual driving experiments while appropriately changing the coefficients of friction of the frictional surface, i.e., the surface roughness and the pressure force for each individual driving apparatus.